1. Field of Invention
The present invention relates to a liquid supplying device. More particularly, the present invention relates to a liquid supplying device equipped with a liquid level sensor suitable for providing a chemical solution to a spin-on-glass (SOG) station.
2. Description of Related Art
FIG. 1 is a sketch showing a conventional liquid supplying device. The liquid supplying device utilizes a capacitor-type of sensor for detecting liquid level. In FIG. 1, the liquid supplying device includes a liquid container 10 placed within an external vessel 11. The liquid container 10 contains a chemical solution 12, and the whole assembly including the solution 12, the liquid container 10 and the external vessel are placed over a capacitor sensor 13.
Chemical solution 12 within the container 10 is able to flow out through a capillary tube to supply a SOG station. As the chemical solution 12 inside the container 10 is gradually used up, the capacitor sensor is able to detect a drop in weight for the whole assembly. Eventually, when the liquid level has dropped below a minimum, a warning signal is emitted so that a user can replenish the chemical solution 12 in the container 10, or simply replace the container 10. Since SOG station must receive the chemical solution 12 continuously, a reliable method of detecting liquid level in a supplying device before the chemical solution 12 runs out is very important.
However, the chemical solution 12 in the liquid container 10 is capable of reacting with air to form crystal particles. These crystal particles add some weight to the whole assembly that includes the chemical solution 12, external vessel 11 and liquid container 10. Therefore, it is possible that even when all the chemical solution 12 inside the container 10 has already been used up, the overall weight is still not low enough to generate any warning signal. Alternatively, if the sensitivity of the capacitor sensor 13 is low, a warning signal may be emitted even though the liquid level in the container 10 is still relatively high.
Consequently, there is an error in detecting the amount of residual solution in the container 10 with a conventional capacitor type of sensor. Hence, problems such as a warning signal not being emitted in time to allow for the replenishing of solution or a false alarm being given when the container is still relatively full can occur from time to time.
In light of the foregoing, there is a need to provide a liquid supplying device equipped with a better level-sensing device.